This invention relates generally to metal refining wherein oxygen is provided to the molten metal from above the surface of the molten metal, and is particularly useful for use with steel refining processes such as the basic oxygen process.
In refining of metals, such as the production of steel using the basic oxygen process (BOP), oxygen is provided into the molten metal bath to react with constituents of the molten metal some of which are incorporated into a molten slag which along with the molten metal comprise the molten bath. These reactions serve to provide heat to the molten metal to help maintain the metal in a molten condition, and also serve to remove unwanted constituents to arrive at the melt chemistry desired for the final product.
The oxygen may be provided to the molten bath from above the molten bath surface, such as in BOP practice, or may be provided to the molten bath from below the molten bath surface, such as in the quick basic oxygen process (Q-BOP) practice and the argon oxygen decarburization (AOD) practice.
The provision of oxygen to the molten bath from above the molten bath surface is less complicated and less expensive than the provision of oxygen to the molten bath from below the molten bath surface because the latter procedure increases refractory wear, requires frequent replacement of the submerged injection devices, e.g. tuyeres, due to the harsh environment created by the submerged oxygen injection and requires the use of an inert or hydrocarbon shroud gas to protect the tuyeres. The high cost is due to the higher refractory consumption, the cost of the shroud gases and replacement tuyeres, and the downtime incurred by the requisite tuyere replacement.
However, top injection of oxygen in metal refining is less effective than bottom injection because less mixing of the molten metal bath occurs with top injection practices. This results generally in lower yields for top blown metal refining processes compared to comparable bottom blown processes. For example, the iron and manganese yield for top oxygen blown converter processes such as the BOP is lower than that for bottom oxygen blown converter processes such as the Q-BOP due to insufficient gas stirring energy for adequate mixing of the metal and slag. In addition, the consumption of aluminum for steel deoxidation is higher for the BOP compared to the Q-BOP due to a higher dissolved oxygen content at the end of the refining process.
One way of addressing this problem is to inject the oxygen from both above and below the molten metal surface. This reduces somewhat the costs associated with oxygen injection from below the molten metal surface and the frequency of tuyere replacement but at the cost of requiring the operation of two separate oxygen provision systems. Another way of addressing this problem is to inject the oxygen into the furnace headspace from only a short distance above the molten metal surface to provide additional mixing of the metal and slag, at least for a portion of the oxygen injection period. However, this practice is still unsatisfactory because of increased wear of the oxygen injection lance.
Inasmuch as the BOP is used to produce about 60 percent of world steel production, any improvement in top oxygen injection processes such as the BOP would be highly desirable.
Accordingly, it is an object of this invention to provide an improved method for refining metal which employs the provision of oxygen to the molten metal from above the molten metal surface and which can provide effective gas stirring energy to the molten metal bath without compromising the integrity of the oxygen injection lance.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for refining metal comprising contacting a molten metal bath containing silicon and carbon for a time sufficient to refine and decarburize said molten metal to a target metal purity with an oxygen-containing gas stream wherein said gas stream contains at least about 80 mole percent oxygen and is provided from a nozzle having an exit diameter d, and wherein said contacting occurs in first and second phases, said first phase constituting the initial about 10 to 90 percent of the total contacting period and being characterized by the gas stream being such as to have a supersonic jet length of less than 30d, having a broad contact area with the molten metal, and being surrounded by a gas shroud comprising a secondary oxygen-containing gas and an inert gas, said first phase conducted until at least 50 percent of the silicon in the molten metal bath has been oxidized; and said second phase constituting substantially the balance of said total contacting period and being characterized by the gas stream being such as to have a supersonic jet length of greater than about 30d, having a smaller contact area with the molten metal, and being surrounded by a flame shroud, said second phase conducted until the molten metal has been decarburized to substantially achieve a target residual carbon level.
As used herein the term xe2x80x9cheadspacexe2x80x9d means the space which is located above the quiescent molten bath surface and below the plane defined by the top opening of the metal refining furnace.
As used herein the term xe2x80x9ccoherent jetxe2x80x9d means a gas stream which has a substantially constant diameter along its length.
As used herein the term xe2x80x9cdecarburizexe2x80x9d means to remove carbon from molten metal by reacting carbon with oxygen to form carbon monoxide or carbon dioxide.
As used herein the term xe2x80x9csupersonic jet lengthxe2x80x9d means the length of a jet from a nozzle wherein its axial velocity is supersonic as measured under ambient atmospheric conditions.
As used herein the term xe2x80x9caxial velocityxe2x80x9d means the velocity of a gas stream at its axial centerline.
As used herein xe2x80x9cjet forcexe2x80x9d means the calculated penetrating force of the jet which is proportional to the product of the gas density and the square of the gas velocity integrated within the area of the gas stream defined by the original nozzle area which is equal to xcfx80d2/4.
As used herein xe2x80x9cambient atmospheric conditionsxe2x80x9d means ambient air with a temperature in the range of zero(0) to one hundred (100) degrees Fahrenheit. For purposes of this invention the gas jets 23 and 30 useful herein are those which satisfy the criteria for axial velocity and jet force retention at a jet length of 30d set forth herein when tested under ambient atmospheric conditions under model test conditions. Gas jets having an axial velocity of less than Mach 1 and a jet force of less than 20% of the original jet force at a jet length of 30d are referred to herein as having a xe2x80x9cbroad contact area with the molten metalxe2x80x9d. Gas jets having an axial velocity of greater than Mach 1 and a jet force of greater than 50% of the original jet force at a jet length of 30d are referred to herein as having a xe2x80x9csmaller contact area with the molten metalxe2x80x9d.
As used herein xe2x80x9cmodel test conditionsxe2x80x9d are as follows. The jets are characterized in an open-air test facility. The jets are formed and injected into the ambient air, where the structure of the jet is probed using a pitot-tube. The pitot-tube measures the dynamic pressure of the flowing gas from which various jet properties can be determined. This probe is capable of moving in three-dimensions, allowing full spatial characterization. Typically, only one of the jets is probed. The un-probed jets are assumed to be identical to the probed jet. The measurements taken are 1) the axial dynamic pressure profile (centerline) and 2) the radial dynamic pressure profile (taken at various axial positions). The Mach number, velocity and force profiles are calculated from the pitot-tube measurements using well-known gas dynamic relationships.